Carabiner hooks are very dangerous for safety's purpose: attached to "D" rings, in some situation they can self open and stay as is until it is too late to realize that you are not link to the boat anymore !

This happen more frequently than we think, not only on "D" rings but on any similar anchoring point....

On our harnesses, tether lines, only double action snaphook both ends are allowed.

Some tether lines are equiped with quick release hook, harness's side to allow disconnection in emergency, but they do not sound very smart to me for following reason:

The hook can eventually be open without being noticed by the user.

In some situation, these quick release hook are very difficult to open and may not garantee 100% opening all the time.

Poorly maintained, the quick relase hook can be jammed by the corrosion...

A good knife that everybody has in the pocket can be a well performing "quick release" tool with a better control instead of this type of hook.

Carabiners are used on board for non-safety related purposes and, generally, with light loads application only.

I have been using my tether for 7 years and in various conditions. My partner's tether is the same set up and equally heavy use. I have never seen a hook open by itself. And I always hook with both hooks in bad weather. (double action hooks here).

Now, at sailor's end, I would opt for a spinnaker type snap shackle - I have read stories of sailors drowned while being towed along the boat - unable to climb back in and unable to unclip.

Dragging behind boat unobserved, cut tether, out of the frying pan into the fire. My lines are rigged so its not possible to hit the water. The tether is just long enough to reach from the deck to my harness. lifelines are run tight to deck from cockpit to bow. I have no clips. Both ends are splices done with 5 tucks with 5/8 nylon three strand. Even if knocked down I feel my chances are better still attached to the boat. I've been left alone in a lifejacket in small seas when a harness on a rented hobie failed. Its something I don't want to repeat in rough conditions.

Design Criteria:
Must remain closed when in normal operation (a positive locking device in ISAF speak) Must open easily one handed with load on;

The race community seems to have settled on the Gibb and Winchard designs to meet these requirements.

I personally find the Winchard design tricky to open with cold hands as you need to squeeze it. I like and use the Gibb design

I would say that a screw gate caribiner is 'positive locking' and so meets the ISAF criteria - not surprisingly - it's designed to carry out the same function.

Having said all that, I have never had or seen a single occasion where the non-positive locking devices have actually opened on their own. However, I have had a few occasions where I thought I'd clipped on, but somehow the webbing wasn't actually captured.

I imagine most of these devices fail from "operator error" than for other reason